The U.S. military has stopped ground shipments of its cargo leaving Afghanistan via a key supply route in Pakistan, citing safety concerns.

U.S. officials said Tuesday that they had ordered truckers under U.S. contract to park at holding areas inside Afghanistan. There have been growing anti-U.S. demonstrations inside Pakistan calling for an end to the American drone program targeting militants.

A Pentagon spokesman said they hope to restart the shipments soon. The cargo includes equipment and other goods being sent home from military units as their numbers are reduced in Afghanistan.